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Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Earnings Per Share [Abstract] |  |  |  |
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts | $ 57.3 | $ 22.7 | $ 13.3 | $ 31.1
Dividends declared net of restricted common share amounts | 0.0 | 0.0 | (3.7) | (4.5)
Total undistributed net losses, net of restricted common share amounts | $ 57.3 | $ 22.7 | $ 9.6 | $ 26.6

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- Definition The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60A -URI http://asc.fasb.org/extlink&oid=109260490&loc=SL5780132-109256 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 66 -URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e2814-109256 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -URI http://asc.fasb.org/extlink&oid=109260490&loc=SL5780133-109256 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 65 -URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e2793-109256 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60 -URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e2740-109256 + Details
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- Definition Equity impact of common stock cash dividends declared by the entity during the period, net of restricted share amounts. + References No definition available. + Details
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- Definition Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares. + References No definition available. + Details
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